The present invention relates to wireless communications systems (apparatus) and methods, and more particularly, to systems and methods for communicating among terminals in wireless communication systems.
Wireless communications systems are commonly employed to provide voice and data communications to subscribers. For example, analog cellular radiotelephone systems, such as those designated AMPS, ETACS, NMT-450, and NMT-900, have been long been deployed successfully throughout the world. Digital cellular radiotelephone systems such as those conforming to the North American standard IS-54 and the European standard GSM have been in service since the early 1990""s. More recently, a wide variety of wireless digital services broadly labeled as PCS (Personal Communications Services) have been introduced, including advanced digital cellular systems conforming to standards such as IS-136 and IS-95, lower-power systems such as DECT (Digital Enhanced Corpses Telephone) and data communications services such as CDPD (Cellular Digital Packet Data). These and other systems are described in The Mobile Communications Handbook, edited by Gibson and published by CRC Press (1996).
Included among the wireless communications systems in use today are low-power private wireless systems that are used to cover a limited geographic area, such as office, a building or a campus. An example of such a private wireless system is the Digital Wireless Office System (DWOS) developed by Ericsson Inc., the assignee of the present invention. As illustrated in FIG. 1, the DWOS 110 is designed for use by wireless terminals 150 that are also configured to access a public cellular network 130, such as a cellular network complying with IS-41, IS-136 or other standards. The DWOS 110 includes a plurality of radio heads 118, i.e., transceiver units that function in a manner similar to base stations of the public cellular network 130. The radio heads 118 are networked with a private cellular radio exchange (CRE) 114 that handles air interface and switching functions, and a mobility server 116 that provides such functions as home location registration, authentication, interfacing the DWOS to the public cellular network 130, as well as other operation and maintenance functions. The DWOS 110 also includes one or more scanning heads 119, receiver units that monitor the radio frequency (RF) environment and determine which frequencies the DWOS 110 can use with the least interference. The DWOS 110 also includes a public exchange (PBX) that connects the radio infrastructure to regular wireline telephones 111 and to the public switched telephone network (PSTN) 120.
The Ericsson DWOS system described is specifically designed to operate according to an IS-136 time division multiple access (TDMA) air interface. An IS-136 TDMA system is implemented by communicating over a plurality of carrier frequency bands during repeating series of time slots, with each repeating series of time slots constituting a xe2x80x9cphysical channel.xe2x80x9d xe2x80x9cLogical channelsxe2x80x9d are mapped onto the physical channels by assigning selected slots of a physical channel to selected channels in repeating frame, superframe and hyperframe structures, in a manner well known to those skilled in the art.
Typically included among the channels are dedicated control channels, including forward (downlink) control channels for conveying information from base stations (or radio heads, in the DWOS) to terminals, and reverse control channels for conveying information from terminals to base stations. The information broadcast on a forward control channel may include such things as an identification of a geographic cell served by a transmitting base station, and associated network identification, system timing and other information needed to access the wireless system. Reverse control channels are typically used for transmitting access requests and page acknowledgments from terminals.
An exemplary slot allocation utilized by wireless systems complying with the IS-136 standard is illustrated in FIG. 2. For groups of three repeating slots on the uplink and downlink carrier frequency bands used by a base station, a xe2x80x9cslot pairxe2x80x9d on one pair of carrier frequency bands is reserved for the provision of a forward (downlink) Digital Control Channel (FDCCH) and a reverse (uplink) DCCH (RDCCH). Other slot pairs may be assigned to Digital Traffic Channels (DTCs), i.e., channels assigned to particular terminals on a dynamic basis for communication of voice or other data.
As illustrated in FIG. 3, an IS-136 FDCCH has a plurality of logical channels mapped thereon, including a multiplexed Broadcast Channel (BCCH) designed to convey information about system configuration and system access rules, and a multiplexed point-to-point short message service (SMS), paging and access response channel (SPACH). The BCCH is further divided into a Fast Broadcast Channel (F-BCCH) for conveying time-critical information such as system identification (ID) and registration information, an Extended Broadcast Channel (E-BCCH) for conveying less time critical information such as neighboring cell lists, and an SMS Broadcast Channel (S-BCCH). The SPACH comprises a short message service channel (SMSCH) for carrying messages, a paging channel (PCH) for conveying system pages, and an access response channel (ARCH) for providing system response to queries from subscriber units and other administration information. A Shared Channel Feedback (SCF) field contains information about the reservation status of an associated Random Access Channel (RACH) on the RDCCH. The RACH is used by terminals to transmit requests to access the wireless system.
A call between terminals 150 in a standard wireless system such as the DWOS 110 of FIG. 1 is typically initiated by one of the terminals 150 transmitting a call origination message that identifies the number the originating terminal desires to call. This origination message is received by one of the radio heads 118, and conveyed to other elements of the DWOS for authorization and traffic channel designation. Once the call request is authorized, the system transmits a traffic channel designation message to the originating terminal, designating a traffic channel for use by that terminal. The DWOS 110 also transmits a page message addressed to the called terminal from selected radio heads 118, typically based on location information maintained by the mobility server 116. The called terminal acknowledges the page by transmitting a page acknowledgment message, which typically is received by more than one of the radio heads. Typically based on relative signal strengths of the acknowledgments received at the radio heads, one of the radio heads is selected to communicate with the called terminal, and an appropriate traffic channel designation message is transmitted to the called terminal, designating a traffic channel for its use.
Communications between the terminals are then conducted over the designated traffic channels. During the call, both voice and control information may be communicated over the designated channels using so-called xe2x80x9cin-bandxe2x80x9d signaling channels mapped onto the designated traffic channels. Signals communicated over the designated traffic channels are also typically monitored for signal quality. This in-band signaling and monitoring is typically used to guide handoffs as terminals 150 move among areas served by different radio heads 118.
This call setup procedure may also be used to setup a group call among more than two terminals. However, the above-described call setup procedure may be too cumbersome for some group call applications, such as emergency police or other safety applications, due to the overhead associated with paging multiple terminals.
In light of the foregoing, it is an object of the present invention to provide efficient apparatus and methods for group calling.
It is another object of the present invention to provide apparatus and methods for group calling in a private wireless system.
These and other objects, features and advantages are provided according to the present invention by wireless communications apparatus and methods in which a group call among terminals is conducted, preferably in a half-duplex manner, over a common traffic channel. For example, in a private wireless system comprising a plurality of synchronized distributed transceiver units or radio heads, voice data can be communicated to terminals over a common physical channel, i.e., frequency/time slot.
A user at an originating terminal of a predefined group may initiate a group call to other terminals in the group by transmitting a group call origination message, preferably one having a special abbreviated format, on a reverse control channel. The call origination message may be transmitted, for example, in response to actuation of a push-to-talk (PTT) device on the originating terminal. The system may respond to the group call origination message by transmitting a group call traffic channel designation message, also preferably having an abbreviated format, to the other terminals in the group over a forward control channel, without paging the other terminals in the group. In response to receipt of the traffic channel designation messages, the terminals may tune to the designated traffic channel and await an in-band confirm message that indicates which of the terminals has been designated to speak on the common channel. When the user at the designated terminal releases the traffic channel, e.g., by releasing the PTT button on the terminal, a user at another terminal in the group can gain control of the common channel by, for example, depressing the PTT button at that terminal and causing a new group call origination message to be transmitted from the terminal.
According to a xe2x80x9ctransmission trunkingxe2x80x9d aspect of the present invention, a terminal in control of the common traffic channel indicates relinquishment of the common channel by transmitting a release message that is communicated over the common channel to the other terminals of the group. When a terminal of the group receives a release message over the common traffic channel, it preferably jumps back to the control channel and actively monitors (e.g., on a slot-by-slot basis) the control channel for new group traffic channel designation messages, thus freeing the common traffic channel for other users. If a terminal in the group receives a new group call traffic channel designation message within a predetermined xe2x80x9chang timexe2x80x9d following the release message, it tunes to the common traffic channel designated in the new designation message and the group call continues over the newly designated common traffic channel. If a terminal in the group fails to receive a new group call traffic channel designation message within the hang time, however, the terminal can assume that the group call has terminated and can transition to a power-saving sleep mode.
Other aspects of the present invention can further increase group call setup efficiency. A special group call xe2x80x9cpagingxe2x80x9d subchannel may be defined that includes slots that repeat multiple times in a xe2x80x9cnormalxe2x80x9d control channel superframe, thus providing a subframe structure within the superframe. The increase frequency of the slots of such a group call paging channel in comparison to normal paging channels can allow group-call enabled terminals to more quickly receive traffic channel designation messages, and can thus reduce group call setup times. Even greater efficiency can be gained by using a compressed addressing scheme that allows a group call origination or a group call traffic channel designation message to be transmitted in a single slot, while maintaining compatibility with terminals that are not group call enabled.
The present invention can provide ways to quickly and efficiently establish and conduct group calls. By using half-duplex communications over a common traffic channel, the overhead associated with normal call setup and control procedures can be avoided. For example, pages and page acknowledgments need not be transmitted to establish a group call, and handoffs need not occur as a terminal moves across a coverage boundary during a call. Addition efficiency may be gained using the special group call paging channels and compressed message formats. In some system, e.g., in private systems in which security is less important, additional economy can be gained by setting up group calls without authentication procedures that are typically used for xe2x80x9cregularxe2x80x9d calls.
In particular, according to the present invention, a group call is conducted in a wireless communications system including a plurality of distributed transceiver units, e.g., radio heads of a private wireless system, each of which are operative to communicate with terminals over a plurality of communications channels. A group call origination message is transmitted from an originating terminal, preferably in response to a user input at the originating terminal, more preferably in response to actuation of a push-to-talk (PTT) device at the originating terminal. The transmitted group call origination message is received at a transceiver unit and, in response, a group call traffic channel designation message addressed to a group of terminals is transmitted from the transceiver units, the transmitted group call traffic channel designation message designating a common traffic channel. Communications then occur among terminals of the group of terminals, preferably in half-duplex mode, over the designated common traffic channel.
According to an aspect of the present invention, a group call traffic channel designation message is transmitted over a control channel which terminals of the group actively monitor when no data is being transferred over the common traffic channel. Data is transmitted from one of the group of terminals over the common traffic channel and received at a transceiver unit. The received data is then transmitted from the plurality of transceiver units over the common traffic channel. Transmission of the received data may then be followed by transmission of a release message from the plurality of transceiver units over the common traffic channel. In response to receipt of the release message, the terminals monitor the control channel for a new group call traffic channel designation message, thus freeing the common traffic channel for other use.
According to another aspect of the present invention, slots assigned to control channels make up a hyperframe comprising a plurality of superframes. Group call enabled terminals monitor a set of periodically occurring slots in a plurality of subframes within a superframe at a terminal, e.g., slots assigned to a special group call paging subchannel of the control channel, such that these terminals can receive traffic channel designation messages in a more timely fashion than ordinary pages.
According to another aspect of the present invention, a group call confirm message is transmitted over the common traffic channel prior to transmission of data received from one of the terminals of the group. The confirm message identifies the originating terminal, thus allowing the terminals to determine which terminal should transmit over the common traffic channel.
According to still another aspect of the present invention, the control channel comprises a series of slots on a carrier frequency band, and a group call traffic channel designation message is transmitted in a single slot assigned to the control channel. The group call traffic channel designation message may be addressed based on a Temporary Mobile System Identification (TMSI) assigned to a terminal by the wireless communications system in order to fit the message into a single slot. Preferably, a least signficant portion of a TMSI field is used to allow group-call enabled terminals to send and receive group call control information in a manner which is compatible with addressing of non-group call enabled terminals.
According to yet another aspect of the present invention, a terminal transitions from active monitoring of the control channel to a sleep mode if a predetermined time interval following receipt of a release message elapses without receipt of a new group call traffic channel designation message on the control channel. The predetermined interval may be indicated in a release message received over the common traffic channel. If a new group call traffic channel designation message designating the same or a new common traffic channel is received at the terminals of the group before lapse of the predetermined interval, however, the group call may resume on the second designated traffic channel. The new group call traffic channel designation message may be transmitted in response to a call origination message transmitted by a terminal of the group, e.g., in response to a user at one of the terminals depressing a PTT button to take control of the group call.
In another embodiment according to the present invention, a wireless communications system comprises means for receiving a group call origination message from an originating terminal. Means are provided, responsive to the means for receiving a group call origination message, for transmitting a group call traffic channel designation message addressed to a group of terminals in response to receipt of the group call origination message, the transmitted group call traffic channel designation message designating a common traffic channel. Means are also provided, responsive to the common traffic channel, for communicating among terminals of the group of terminals over the designated common traffic channel. The means for receiving a group call origination message, the means for transmitting a group call traffic channel designation message and the means for communicating among terminals preferably comprise a plurality of distributed transceiver units, a cellular radio exchange connected to each of the plurality of distributed transceiver units, and a mobility server connected to the cellular radio exchange.
In another embodiment according to the present invention, a terminal for use in wireless communications systems includes means for receiving a group call traffic channel designation message addressed to the terminal, the received group call traffic channel designation message designating a common traffic channel; and means, responsive to the means for receiving, for communicating with a plurality of other terminals over the designated common traffic channel. The terminal may further comprise means for transmitting a group call origination message responsive to a user input.
Improved methods, systems and terminals for conducting wireless group calls may thereby be provided.